1. Technical Field
The present invention relates to processes for producing titanium alloys and more particularly to processes for melting and casting ruthenium and iridium containing titanium alloys.
2. Background Information
Platinum-group metals have been incorporated into various commercial titanium alloys over the past forty years primarily for the purpose of improving and expanding corrosion resistance. The most prominent examples are the Tixe2x80x94Pd binary alloys, ASTM Grades 7 and 11 (Ti-0.15 wt. % Pd), and Grades 16 and 17 (Ti-0.05 wt. % Pd) which have been widely used in severe, corrosive service in the chemical process industry. This minor 0.04-0.25 wt. % Pd addition to etitanium and its alloys dramatically enhances its resistance to dilute reducing acid (HCl, H2SO4) media, as well as to crevice corrosion in hot chloride or halide salt solutions (brines).
The formulation, melting, and casting of these commercial Tixe2x80x94Pd alloys is routinely and readily accomplished using the same standard, established methods/practices used for unalloyed titanium and most common titanium alloy mill products and components. This includes direct blending or addition of palladium metal powder to titanium metal sponge and/or revert, and subsequent melting via vacuum arc remelting (VAR), electron beam (EBM) melting, plasma arc melting (PAM), and VAR skull melting (for shaped castings) methods. These classic methods have produced acceptable, very chemically homogeneous ingots and castings stemming from palladium""s slightly lower melting point than titanium (Table 1).
As the market price of palladium metal continued to rise, such as the xe2x80x2$140 to over $600 perTroy oz. increase in 1999, the price of Tixe2x80x94Pd mill products became too exorbitant for most chemical process applications, and dramatically thwarted its selection and use. In an effort to provide lower cost substitutes for various expensive Tixe2x80x94Pd alloys, various Tixe2x80x94Ru binary alloys have been developed, such as ASTM Grades 26 and 27 (Ti-0.1 wt. % Ru). Such Tixe2x80x94Ru alloys often exhibit comparable corrosion resistance and mechanical/physical properties to the Tixe2x80x94Pd alloys. Since these alloys were formulated using ruthenium metal additions which are currently on the order of one-sixth the price of palladium, these Tixe2x80x94Ru alloys also offer substantial cost savings over similar Tixe2x80x94Pd mill products.
Over the past decade several commercial high-strength ruthenium-containing alpha-beta and beta titanium alloys have been developed and qualified. These alloys include ASTM Grades 29 (Ti-6Al4V-0.1 Ru), 28 (Ti-3Al-2.5V-0.1 Ru), and Ti-3Al-8V-6Cr4Zr-4Mo-0.1 Ru alloys, which are commercially utilized in high temperature, corrosive energy industry service, such as geothermal brine production well casing, oil/gas production tubulars and offshore riser components.
At the current price of $415.00/Troy oz., iridium metal is also a more cost effective alloy addition than Pd or Pt. Since iridium exhibits similar electrocatalytic behavior as Pt, the possibility exists that it can be added to titanium alloys in amounts as low as 0.05 wt. % nominally to enhance corrosion performance.
Although ruthenium (Ru) and iridium (Ir) are desirable, lower cost means of upgrading titanium alloy resistance than palladium (Pd), traditional methodologies forformulation and melting of Ru and Ir-containing titanium alloys based on direct elemental Ru or Ir metal additions to titanium melts pose distinct concerns and difficulties for achieving alloy compositional homogeneity. Specifically, ruthenium and iridium metals represent a xe2x80x9crefractoryxe2x80x9d, difficult-to-melt-in additions to titanium using traditional melting processes such as VAR and EBM/PAM hearth processes. Attempts to utilize accepted methods of direct addition of elemental powder employed successfully with Pd metal additions over the years, has been known to result in Ru macrosegregation and serious inhomogeneities with respect to Ru content in Ti-0.1Ru alloy ingots and castings. Although this gross inhomogeneity in Ru or Ir can be minimized in ingots/castings by direct Ru or Ir addition to individual Ti compacts, use of fine Ru or Ir powders, and/or improved blending with sponge granules, it is difficult to avoid. In fact, this problem is aggravated and enhanced when producing Ru- or Ir-containing Ti alloy heats via EBM or PAM hearth processes.
This difficulty in achieving homogeneous titanium melts using ruthenium metal additions primarily stems from ruthenium""s and iridium""s exceptionally high melting point and density, which are roughly 675xc2x0 C. (1215xc2x0 F.) and 781xc2x0 C. (1406xc2x0 F.) above that of pure titanium metal, respectively (see Table 1). In contrast Pd metal melts xe2x80x2110xc2x0 C. (198xc2x0 F.) below that of titanium. Furthermore, Ru and Ir metals both possess a substantially higher latent heat of fusion compared to either Ti or Pd metal (Table 1), which further inhibits their ability to transition from solid particle to liquid form (melt). On the other hand, Ru and Ir have a comparable specific heat but much higher thermal conductivity than Pd, which can be expected to partially counteract these refractory Ru and Ir properties which retard melt kinetics.
Two key factors for successful melting of any alloy addition into a titanium alloy melt are: 1) achieving sufficient temperature to exceed the melting point of the addition, and 2) allowing sufficient residence time of the particle addition within the high energy source and the superheated Ti melt to fully melt/dissolve the added particles. In the case of VAR melting of titanium from consumable electrodes, the pre-heated electrode compact adjacent to the electric arc becomes very hot and can pre-sinter compacted Ru and/or Ir metal powders into more difficult-to-melt, larger, consolidated clumps exhibiting low surface-to-volume ratio and a larger thermal mass. Pre-blending Ti compacts using very fine Ru or Ir powder helps, but classification and settling of the fine Ru or Ir powder mixed with coarse Ti metal sponge and other master alloy granules is inevitable and unavoidable prior to compacting. Although high temperatures well above ruthenium""s and iridium""s melting point are achieved within the electric arc, residence time is very short and actual Ru or Ir particle size mass can be too large to ensure total melting of refractory Ru or Ir additions. With ruthenium""s and iridium""s much higher density (Table 1) compared to Ti, these heavy unmelted Ru and Ir particles tend to settle rapidly within the liquid Ti melt causing macrosegregation and gross chemical inhomogeneity in ingots and castings.
Although Ru or Ir powder pre-sintering within electrode compacts is not a relevant concern in cold-hearth melt processes, the concern for incomplete Ru and/or Ir particle melting is even greater in these types of processes. This concern stems from:
1) The minimal, extremely short exposure of the solid Ti/Ru or Ti/Ir metal input mixture to either the electron beam or plasma arc as they raster across the metal and/or liquid melt surface.
2) The relatively low superheat in the titanium melt pool, involving temperatures well below ruthenium""s or iridium""s high melting point, and
3) The high density of unmelted Ru and Ir metal particles which promotes rapid settling (gross macro-segregation) of Ru and Ir particles within the very shallow Ti melt pool as it flows over the cold hearth.
It is an object of the present invention to provide an effective means for using Ru and Ir as a cost means of upgrading titanium alloy corrosion performance to achieve the required reducing acid, crevice, and stress corrosion.
The present invention is an improved process for successful and homogeneous incorporation of ruthenium into titanium and titanium alloy melts, ingots, and castings via traditional and contemporary melting processes (e.g., VAR and cold-hearth) has been developed. This is achieved through the use of low-melting point Tixe2x80x94Ru binary master alloy within the general composition range of xe2x89xa645 wt. % Ru, and with a preferred composition of  greater than 15 wt. % and xe2x89xa640 wt. % Ru. For iridium a low melting point Tixe2x80x94Ir binary master alloy containing xe2x89xa661 wt. % Ir, with a preferred range of  greater than 20 and xe2x89xa658 wt. % Ir. Desirable features as a master alloy for titanium melt processes are outlined in Table 4. Primary features are their lower melting point than pure titanium, lower density than pure Ru and Ir, and the ability to be readily processed into granular or powder forms.